Continuous Renal Replacement Therapy (CRRT) requires precise calculation of fluid dynamics to ensure patient safety and therapy efficacy. The Ultrafiltration Rate (UFR) determines how much fluid is removed from the patient's blood. Correctly calculating the Net UFR (for fluid balance) and monitoring the Filtration Fraction (for filter patency) are critical skills for ICU clinicians.
This guide explains the formulas used to calculate the Net Ultrafiltration Rate required to meet a fluid balance goal, and the Filtration Fraction to prevent clotting.
1. Calculating Net Ultrafiltration Rate (Net UFR)
The Net UFR is the rate at which you program the CRRT machine to remove fluid specifically to achieve a negative or neutral fluid balance. This is separate from replacement fluid which is given to clean the blood.
Formula:
Net UFR = Total Patient Intake – Total Non-CRRT Output – Target Hourly Balance
Where:
Total Intake: IV fluids, medications, nutrition, blood products (mL/hr).
Target Balance: The desired fluid change per hour (e.g., -100 mL/hr).
Example Calculation
A patient has an intake of 250 mL/hr and urine output of 50 mL/hr. The physician orders a net negative balance of -100 mL/hr.
$$Net\ UFR = 250 – 50 – (-100) = 300\ mL/hr$$
You would set the machine's fluid removal rate (often labeled "Net Removal" or "Patient Removal") to 300 mL/hr.
2. Calculating Filtration Fraction (FF)
While Net UFR manages fluid overload, the Filtration Fraction determines the safety of the therapy. It represents the percentage of plasma water removed from the blood as it passes through the filter. If FF is too high (>20-25%), the blood thickens excessively inside the filter, leading to clotting.
Step 1: Calculate Total Ultrafiltration Rate ($Q_{total\_uf}$)
In CVVH (hemofiltration) or mixed modes:
$$Q_{total\_uf} = Net\ UFR + Replacement\ Fluid\ Rate + Pre\ Blood\ Pump\ Rate (PBP)$$
Step 2: Calculate Plasma Flow Rate ($Q_p$)
Since filters only filter plasma, not red blood cells, you must convert whole blood flow ($Q_b$) to plasma flow.
$$Q_p (mL/hr) = Q_b (mL/min) \times 60 \times (1 – Hct)$$
(Note: Hct must be in decimal form, e.g., 0.30 for 30%)
Clinical Guidelines for Rates
Parameter
Typical Range
Safety Note
Net UFR
0 – 500 mL/hr
Monitor for hypotension if > 1.5-2.0 mL/kg/hr.
Filtration Fraction
< 20% – 25%
> 25% significantly increases clotting risk.
Blood Flow ($Q_b$)
150 – 300 mL/min
Higher flow helps reduce Filtration Fraction.
CRRT Calculator
Calculate Net UFR & Filtration Fraction
1. Fluid Balance Targets
mL/hr
IVs, meds, feeds, etc.
mL/hr
Urine, drains, stool
mL/hr
Use negative (-) for removal
2. Machine Settings (Safety)
mL/hr
Total convective volume
mL/min
%
Set Net UFR (Patient Removal)0 mL/hr
Total Ultrafiltration Rate0 mL/hr
Filtration Fraction (FF)0%
function calculateCRRT() {
// Get Input Values
var intake = parseFloat(document.getElementById('totalIntake').value);
var output = parseFloat(document.getElementById('nonCrrtOutput').value);
var target = parseFloat(document.getElementById('targetBalance').value);
var replacement = parseFloat(document.getElementById('replacementFluid').value);
var bloodFlow = parseFloat(document.getElementById('bloodFlow').value);
var hct = parseFloat(document.getElementById('hematocrit').value);
// Validation: Treat NaNs as 0 or require input?
// For smoother UX, we treat empty fluid fields as 0, but blood flow/Hct need defaults if empty.
if (isNaN(intake)) intake = 0;
if (isNaN(output)) output = 0;
if (isNaN(target)) target = 0; // Assume neutral if blank
if (isNaN(replacement)) replacement = 0;
// Critical values for FF calculation
var hasSafetyData = true;
if (isNaN(bloodFlow) || bloodFlow <= 0) hasSafetyData = false;
if (isNaN(hct) || hct = 100) hasSafetyData = false;
// 1. Calculate Net UFR
// Formula: Net = Intake – Output – Target
// If Intake is 100, Output is 0, Target is -50.
// Net = 100 – 0 – (-50) = 150. Correct.
var netUFR = intake – output – target;
// 2. Calculate Total Ultrafiltration
// Total UF = Net UFR + Replacement
var totalUFR = netUFR + replacement;
// 3. Calculate Filtration Fraction
var ff = 0;
var ffText = "–";
if (hasSafetyData) {
// Convert Qb (mL/min) to mL/hr: Qb * 60
var qbHourly = bloodFlow * 60;
// Calculate Plasma Flow: Qb_hr * (1 – Hct_decimal)
var plasmaFlow = qbHourly * (1 – (hct / 100));
// FF = (Total UF / Plasma Flow) * 100
if (plasmaFlow > 0) {
ff = (totalUFR / plasmaFlow) * 100;
ffText = ff.toFixed(1) + "%";
}
}
// Display Results
document.getElementById('resultsArea').style.display = "block";
document.getElementById('resNetUFR').innerText = netUFR.toFixed(0) + " mL/hr";
document.getElementById('resTotalUFR').innerText = totalUFR.toFixed(0) + " mL/hr";
document.getElementById('resFF').innerText = ffText;
// Safety Logic
var alertBox = document.getElementById('safetyAlert');
alertBox.style.display = "block";
if (hasSafetyData) {
if (ff > 25) {
alertBox.style.backgroundColor = "#ffebee";
alertBox.style.color = "#c62828";
alertBox.innerHTML = "⚠️ High Clotting Risk: FF is > 25%.Consider increasing Blood Flow (Qb), reducing Replacement fluid, or adding Pre-filter replacement.";
} else if (ff > 20) {
alertBox.style.backgroundColor = "#fff8e1";
alertBox.style.color = "#f57f17";
alertBox.innerHTML = "⚠️ Warning: FF is > 20%. Monitor filter pressures closely.";
} else {
alertBox.style.backgroundColor = "#e8f5e9";
alertBox.style.color = "#2e7d32";
alertBox.innerHTML = "✅ Safe Range: Filtration Fraction is within optimal limits (<20%).";
}
} else {
alertBox.style.backgroundColor = "#e3f2fd";
alertBox.style.color = "#0d47a1";
alertBox.innerHTML = "Note: Enter Blood Flow and Hematocrit to see Filtration Fraction safety analysis.";
}
// Negative UFR Logic Check (Impossible machine setting)
if (netUFR < 0) {
document.getElementById('resNetUFR').style.color = "#c62828";
document.getElementById('resNetUFR').innerText = netUFR.toFixed(0) + " mL/hr (Invalid)";
alertBox.style.backgroundColor = "#ffebee";
alertBox.style.color = "#c62828";
alertBox.innerHTML = "Error: Calculated Net UFR is negative. The patient requires fluid administration, not removal, to meet this target.";
} else {
document.getElementById('resNetUFR').style.color = "#212529";
}
}